As known in the art, high selectivity catalysts (HSCs) for the epoxidation of ethylene refer to those catalysts that possess selectivity values higher than high activity catalysts (HACs) used for the same purpose. Both types of catalysts include silver as the active catalytic component on a refractory support (i.e., carrier). Typically, one or more promoters are included in the catalyst to improve or adjust properties of the catalyst, such as selectivity.
Generally, HSCs achieve the higher selectivity (typically, in excess of 87 mole % or above) by incorporation of rhenium as a promoter. Typically, one or more additional promoters selected from alkali metals (e.g., cesium), alkaline earth metals, transition metals (e.g., tungsten compounds), and main group metals (e.g., sulfur and/or halide compounds) are also included.
There are also ethylene epoxidation catalysts that may not possess the selectivity values typically associated with HSCs, though the selectivity values are improved over HACs. These types of catalysts can also be considered within the class of HSCs, or alternatively, they can be considered to belong to a separate class, e.g., “medium selectivity catalysts” or “MSCs.” These types of catalysts typically exhibit selectivities of at least 83 mole % and up to 87 mole %.
In contrast to HSCs and MSCs, the HACs are ethylene epoxidation catalysts that generally do not include rhenium, and for this reason, do not provide the selectivity values of HSCs or MSCs. Typically, HACs include cesium (Cs) as the only promoter.
It is well known that with use of a catalyst, the catalyst will age (i.e., degrade) to a point until use of the catalyst is no longer practical. For obvious reasons, there is a continuous effort to extend the useful lifetime (i.e., “longevity” or “usable life”) of catalysts. The useful lifetime of the catalyst is directly dependent on the stability of the catalyst. As used herein, the “useful lifetime” is the time period for which a catalyst can be used until one or more functional parameters, such as selectivity or activity, degrade to such a level that use of the catalyst becomes impractical.
It is known in the art that, while the selectivity of HSCs is generally acceptable to the industry, their useful lifetime can use improvement. For example, while HACs typically last between 24 and 36 months, HSCs tend to be operated for less than 24 months, often less than 12 months, typically due to an unacceptable loss of selectivity.
Stability of the catalyst has largely been attributed to various characteristics of the carrier. Some characteristics of the carrier that have undergone much research include surface area, porosity, and pore volume distribution, among others.
The most widely used formulation for the carriers of ethylene epoxidation catalysts are those based on alumina, typically α-alumina. Much research has been directed to investigating the effect of the alumina composition for improving stability and other properties of the catalyst. The preparation and modification of alumina carriers for enhancing ethylene epoxidation catalyst performance are described, for example, in U.S. Pat. Nos. 4,226,782, 4,242,235, 5,266,548, 5,380,697, 5,597,773, 5,831,037 and 6,831,037 as well as in U.S. Patent Application Publication Nos. 2004/0110973 A1 and 2005/0096219 A1. In particular, U.S. Pat. No. 5,395,812 discloses coating the outer surface and surface of pores therein with an amorphous silica-alumina mixture in order to improve, inter alia, the lifetime of a silver-based ethylene epoxidation catalyst.
However, there remains a need in the art for further improvements in the stability of ethylene epoxidation catalysts. There is a particular need for improving the stability of such catalysts by modifying the carrier by means that are facile and financially feasible.